ABCG2 and neoplasm: ABC transporters are mainly in charge of the secretion and excretion of endogenous or exogenous substances from cells to maintain a normal physiologic homeostasis, but in tumor cells, the superabundant ABC transporters will recognize exogenous chemotherapeutic agents and pump them out to avoid therapeutic cytotoxicity, which then leads to resistance to many drugs and dismal prognosis in patients [37].